Light-emitting diodes are used in display units of numerous electronic apparatuses owing to their low power consumption, long life, and small size. Recently, the light-emitting diode is widely used in personal products, such as mobile telephones (for button illumination) and the like. Early light-emitting diodes had a low emission intensity and a longer emission wavelength than that of green light and therefore had only limited applications. In recent years, light-emitting diodes made of nitride semiconductors have been put into practical use, and various light-emitting diodes having a high luminance have been mass-produced. Therefore, these light-emitting diodes are applied to light sources for outdoor display apparatuses, automotive headlamps, indoor illumination, and the like.
The nitride semiconductors are gallium nitride (GaN)-based materials. In particular, the emission wavelength can be controlled by adding indium (In) to GaN. This property contributes to the commercialization of high-luminance blue-light-emitting diodes. In such nitride light-emitting diodes, a substrate made of sapphire (single crystal Al2O3), silicon carbide (SiC), or the like is employed for growth of the nitride semiconductor. The sapphire substrate is most commonly used for crystal growth of the nitride semiconductor. Because sapphire is transparent to emission light, the emission light can be advantageously extracted through the back surface of the element as well. However, because sapphire is an insulator, the electrodes necessarily need to be provided on the upper surface (nitride semiconductor side), and also, because sapphire is considerably rigid, it is difficult to process sapphire, which is a problem. On the other hand, SiC has a lattice constant close to that of GaN, and therefore, a high-quality crystal of the nitride semiconductor can be advantageously grown. However, the SiC substrate is expensive, inevitably leading to a high cost per chip.
Also, in recent years, some self-supporting substrates made of gallium nitride (GaN) having a sufficiently low dislocation density have been put into practical use as substrates for growth of the nitride semiconductor. These substrates allow not only heteroepitaxial growth, but also homoepitaxial growth, on the sapphire and SiC substrates, whereby crystal growth having a low dislocation density can be achieved. Therefore, the substrate is widely used in blue-violet nitride semiconductor laser diodes which are employed as light sources for next-generation optical disk apparatuses. On the other hand, attempts are being made to achieve mass-production of a nitride light-emitting diode employing the GaN self-supporting substrate. However, the GaN self-supporting substrate is considerably expensive and therefore the cost per chip is also high compared to the SiC substrate.
Moreover, in recent years, attempts have been made to employ a substrate made of silicon (Si) for growth of the nitride semiconductor (see, for example, NON-PATENT DOCUMENT 1). Inexpensive Si substrates in the form of a wafer having a diameter of up to as large as about 30.5 cm (=12 inches) are available, and therefore, the cost per chip can be reduced to a relatively low level. If a semiconductor laser diode can be implemented on the silicon substrate, the cost per chip can be reduced by at least one order of magnitude compared to the current cost.